For golfers, it is an eternal theme to extend flight distance of a ball and shoot the ball at an aimed direction and angle. Therefore, it is important to use a golf club suited for one's own swing. Selecting a golf club suited for a golfer is generally referred to as fitting.
With regard to this fitting, the present applicants have proposed a method of having a golfer actually hit a ball with a golf club whose grip is attached with a sensor capable of measuring angular velocity and acceleration about three axes, and analyzing a swing of the golfer from the obtained measurement result (Japanese Patent Application No. 2011-196727; hereinafter, also referred to as a “first prior invention”). With this method, a golf club that matches a golfer can be selected by properly analyzing a swing.
In the first prior invention, in order to classify a golfer's swing, impact time is extracted from a chronological waveform of angular velocity about a shaft axis measured using a sensor. Specifically, in a chronological waveform of an angular velocity of a golf swing including the moment of impact, a time at which the angular velocity about the shaft axis (z-axis) becomes a maximum is determined as a provisional impact time. Then, for the purpose of extracting the impact time with better accuracy, in around the provisional impact time (a time duration from a clock time before the provisional impact time by a predetermined time to a clock time after the provisional impact time by a predetermined time), a time that is the earlier between a time at which the angular velocity ωx about the x-axis becomes a minimum and a time at which an acceleration about the z-axis becomes a minimum is set as an impact time.
When a golfer hoping for a fitting test-hits a ball, depending on the place (hit-spot) where a club face hits a ball, there are cases as shown in FIG. 23 where the angular velocity ωz about the shaft axis becomes a maximum not at the time of impact but after impact. FIG. 23 and a later described FIG. 24 each show one portion (around the impact) of a chronological waveform of the angular velocity ωz about the shaft axis, and in these figures, the horizontal axis represents data point numbers from the start of data acquisition and the vertical axis represents angular velocity (deg/s). Since data sampling frequency in the illustrated examples is 1000 Hz, a single data point is 1 msec. In the example in FIG. 23, although point A is the proper impact time that should be extracted, there is a possibility that a point B, at which the angular velocity ωz is a maximum, is falsely recognized as the impact time.
In addition, in cases such as when a swing is performed at an angular velocity exceeding a measurable range of a gyro sensor used for the measurement, there are cases where over range of the sensor occurs as shown in FIG. 24. Also in this case, although point C is the proper impact time that should be extracted, there is a possibility that a point D, at which the angular velocity ωz is a maximum in the range, is falsely recognized as the impact time.
If a correct impact time is not determined, it is conceivable that the accuracy of fitting based on a swing of a golfer analyzed using the impact time will be influenced.
In response, the present applicants have proposed a method for extracting impact time in a golf swing, enabling improvement of accuracy in determining impact time (Japanese Patent Application No. 2012-081773; hereinafter, also referred to as a “second prior invention”). In the second prior invention, it is judged whether an angular velocity ωz about a shaft axis is over range, and a method for determining the impact time is changed depending on the judgment result.
In the second prior invention, a predetermined provisional impact time is configured in accordance with whether or not over range has occurred, three candidates of impact time are acquired from respective chronological waveforms of angular velocities about three axes by using the provisional impact time as a reference, and impact time is determined from these three candidates in accordance with a predetermined determination criterion. As a result, impact time can be determined with fine accuracy by removing the effect of the above described over range etc.